1. Field of the Invention
This invention concerns the production of paper and paperboard.
2. Brief Description of Related Art
Conventional paper or paperboard manufacture involves forming a fibrous stock containing additives such as pigments, fillers and sizing agents and dewatering the stock on a metal or fabric wire to form the basis for the paper or board sheet. Such processes have been subject to the conflicting requirements that ready drainage of the stock should occur and that there should not be undue loss of additives and of fibre from the stock in the course of drainage, that is, that the retention of such additives and fibre on the wire should be high. This acts not only to give a saving in raw material costs and a reduction in the energy required to dry the sheet but also reduces effluent treatment requirements as a result of a lower content of suspended solids, and lower COD and BOD loadings, in the purge water. Sheet formation and surface properties may also be improved. There have been many attempts to optimise drainage and retention properties by the use of combinations of additives, which include polyelectrolytes such as high molecular weight polyacrylamide and its copolymers, which act as flocculants.
It has been proposed to use colloidal swelling clays in conjunction with the high molecular weight, relatively low charge density polyacrylamides which have traditionally been used as flocculants, which may be non-ionic, anionic or cationic in nature and may be selected to suit the charge demand of the stock.
U.S. Pat. No. 3,052,595, for example, discloses the addition of bentonite to filled stock followed by an acrylamide homopolymer or copolymer which may include at most about 15% by weight of a functional comonomer which may be anionic or cationic in nature, corresponding to a charge density of at most about 2 m.eq./g. The affect of the above combination is that the polymer and the bentonite "are mutually activating whereby increased retention of the filler in the paper web and decreased turbidity of the resulting white water are obtained".
More recently, EP-A-0017253 disclosed that the fibre retention and dewatering properties of substantially filler-free stocks may be improved dramatically by including in the stock a high molecular weight polyacrylamide and a bentonite-type clay. The polyacrylamide may contain not more than 10% of either cationic or anionic units and is limited thereby to low charge density material.
U.S. Pat. Nos. 4,753,710 and 4,913,775 disclose a process, the Hydrocol process, comprising adding a high molecular weight linear cationic polymer to thin stock to form large flocs, subjecting the flocculated suspension to significant shear and adding bentonite to the sheared suspension. It is explained that the effect of shearing is to break the flocs down into microflocs which are sufficiently stable to resist further degradation. A further and more detailed explanation of the Hydrocol process mechanics is provided by the inventor in TAPPI Proceedings, 1986 Papermakers Conference, pages 89-92. On page 90, it is noted that the inventor states that the "key to achieve supercoagulation is to balance the charges and surface area of the pre-treated stock with the charge and surface area of the secondary addition". Furthermore, on column 10, lines 26-43, of U.S. Pat. No. 4,753,710 and column 10, lines 59-66, of U.S. Pat. No. 4,913,775, it is stated that in the process it is essential to use a cationic polymer as the flocculant, rather than a non-ionic or anionic polymer.
It is an object of the present invention to provide a process for making paper and paperboard in which the drainage and retention properties of the stock are modified.